1. Field of the Invention
The present invention relates to an optical recording medium in which preformat data are recorded and more, particularly to an optical recording medium of using a land/groove recording method. The present invention also relates to a method and an apparatus for reproducing the preformat data recorded in the optical recording medium.
2. Description of Related Art
As a main file in the multi-media era, there have been increasing expectations of a significant role for optical disks having a large capacity and capable of media exchange. The amount of data such as image data is enormous, and the recording capacity of an optical disk is desired to be further increased. As methods for increasing the recording capacity of an optical disk, a magnetic super-resolution reproducing method is regarded as a useful system for increasing the recording density in the tracking direction, and a land/groove recording method has been developed as a method for increasing the recording density in the radial direction.
An optical disk is provided with grooves for guiding light irradiating the optical disk for track control. In a general land recording method, data are recorded only in lands formed between the grooves, but in the land/groove recording method, the data are recorded in both the lands and the grooves. Thus, a track pitch is minimized, thereby increasing the recording capacity.
In each sector of an optical disk, preformat data such as ID data corresponding to the address of the sector are recorded. Since the ID data are recorded in the form of pits, they cannot be recorded in the grooves. Therefore, a technique has been proposed in which no groove is formed in a part of a groove portion for recording the ID data but pits are formed therein instead.
FIG. 1 is a schematic partial plan view for showing the configuration of conventional pre-pits by using this technique. As is shown in FIG. 1, grooves and lands are alternately formed as a groove 61, a land 62 and a groove 63, and part of the grooves 61 and 63 are respectively provided with pre-pit areas formed at the same height as the land 62, and no groove is formed in the pre-pit areas. In the pre-pit areas of the groove 61 and 63, ID data P.sub.1 and P.sub.3 are respectively recorded. In an area of the land 62 sandwiched between the pre-pit areas, ID data P.sub.2 are recorded. In such an optical disk, the distance among the ID data P.sub.1, P.sub.2 and P.sub.3 is smaller than that in an optical disk of the general land recording method. Therefore, cross talk occurring in the reproduction of the ID data is increased, and the track pitch cannot be made small.
In order to solve this problem, another type of pre-pits shown in FIG. 2 has been proposed. FIG. 2 is a schematic partial plan view for showing the configuration of the conventional pre-pits. As is shown in FIG. 2, in pre-pit areas of grooves 61 and 63, ID data P.sub.1 and P.sub.3 are recorded. A pre-pit area of a land 62 is formed at a position shifted from the pre-pits areas of the grooves 61 and 63 in the tracking direction, and ID data P.sub.2 are recorded therein. The remaining configuration is the same as that of FIG. 1, and the description is omitted.
Since the ID data P.sub.2 of the land 62 is recorded at the position shifted from the positions of the ID data P.sub.1 and P.sub.3 of the grooves 61 and 63, the cross talk can be decreased and the track pitch can be made small. However, the pre-pit areas occupy a larger area than that in general pre-pit areas, which can prevent high density data recording.
Japanese Patent Application Laid-Open Nos. 4-195939 (1992) and 7-29186 (1995) disclose optical data recording media for overcoming the aforementioned problems. FIG. 3 is a schematic partial plan view of the optical disk disclosed in Japanese Patent Application Laid-Open No. 4-195939 (1992), wherein grooves are formed in the entire areas of grooves 61 and 63. A land 62 is disposed between the grooves 61 and 63, and in the boundary between the groove 61 and the land 62, pre-pits P.sub.4 are formed. When a pitch between the grooves is assumed to be x, the center line of the pre-pits P.sub.4 is shifted from that of the groove 61 toward the land 62 by a distance of substantially x/4. Also, in the boundary between the groove 63 and a land 64, pre-pits P.sub.5 are similarly formed.
In reproducing the preformat data from this optical disk, the pre-pits P.sub.4 are commonly reproduced in the reproduction of the groove 61 and the land 62. The groove 61 is distinguished from the land 62 based on the polarity of a tracking control signal. By commonly using the same pre-pits between the land and the adjacent groove in this manner, the cross talk can be decreased, and there is no need to wastefully provide an area for the pre-pits.
In the optical disk shown in FIG. 3, since the pre-pits are formed in the boundary between the projected land and the recessed groove, the shape of each pre-pit is asymmetric, and hence, an accurate pit signal cannot be obtained. A recording medium for overcoming this problem is disclosed in Japanese Patent Application Laid-Open No. 7-29186 (1995). FIG. 4 is a schematic partial plan view of the optical disk disclosed in Japanese Patent Application Laid-Open No. 7-29186 (1995), wherein reference numerals 61 and 63 denote grooves and a reference numeral 62 denotes a land between the grooves. Part of the grooves 61 and 63 are provided with pre-pit areas 65 and 66 having no grooves and formed at the same height as the land 62. In the boundary between the groove 61 and the land 62, pre-pits P.sub.4 are formed. When a pitch between the grooves is assumed to be x, the center line of the pre-pits P.sub.4 is shifted from that of the groove 61 toward the land 62 by a distance of substantially x/4. Also in the boundary between the groove 63 and the land 64, pre-pits P.sub.5 are similarly formed.
In the optical disk having the aforementioned configuration, since the pre-pits are formed in the areas at the same height as the land, the shape of each pre-pit is symmetrical, from which an accurate pit signal can be obtained. However, the land is distinguished from the groove based on the polarity of a tracking control signal as described above, and the area where the pre-pits are formed is at the same height as the land, namely, has no groove thereon. This can disturb the tracking control signal. As a result, it is difficult to accurately distinguish between the land and the groove on the basis of the polarity of a tracking control signal. In addition, when a light beam crosses the tracks at a high speed during a seeking operation, the disturbance of a tracking control signal can result in error reading of a track count value.
Furthermore, since the thus formed preformat area is at the same height as the land, a larger quantity of light is reflected by the preformat area in the reproduction as compared with another data recording area. For example, when data are recorded in the optical disk by the magneto-optical recording method, a light quantity is largely changed when an area irradiated with the light beam moves from the preformat data area to the magneto-optical recording area. This disturbs a reproducing signal, and hence, the magneto-optically recorded data cannot be accurately reproduced.
Additionally, for the production of a substrate bearing such pre-pits, two beams are used for the formation of the groove and one beam is used for the formation of the pits. Thus, three beams in total are required for the production, which requires a large scale apparatus and labor for the fabrication of a stamper for the optical medium.